1. Field of the Invention
The present invention relates to a process for forming an oxide film on an underlying surface adapted for film formation thereon, according to a radio frequency magnetron sputtering method using an oxide target(s).
2. Discussion of Background Art
A radio frequency (RF) magnetron sputtering method has been known as a means of forming a thin metal or oxide film. In this RF magnetron sputtering method, the so-called industrial frequency of 13.56 MHz is used as the excitation frequency (or radio frequency) for ionization of a sputtering gas.
There have been reported cases of thin metallic film formation where a metal film uniform in a sense of freedom from different orientations is formed using an excitation frequency exceeding the industrial frequency of 13.56 MHz [for example, S. Broydo and C. B. Osburn: "1st. Int. Symp. on Ultra Large Scale Integration Science and Technology 1987", S. Broydo and C. B. Osburn, 87-11, p. 574, "The Electrochemical Society Inc., Penington, (1987)", and T. Ohmi et al., "Room-temperature copper metallization for ultralarge-scale integrated circuits by a low kinetic-energy particle process", Applied Physics Letter, vol. 52, p. 2236 (1988). While attempts to improve the properties, such as of composition, uniformity of film thickness, surface morphology (flatness) and crystallinity (orientation), of a thin oxide film by utilizing film formation parameters such as substrate temperature, gas pressure and target composition have been, there have been no reported cases where an excitation frequency higher than the industrial frequency of 13.56 MHz was used to form a thin oxide film using an oxide target(s).
When a thin oxide film is formed through sputtering of an oxide target with an ionized sputtering gas, according to the RF magnetron sputtering method using the industrial frequency of 13.56 MHz, the chemical composition of the thin oxide film greatly differs from the chemical composition of the oxide target. The term "oxide" used herein is intended to encompass compound oxides, containing at least two metallic elements, are particularly referred to as in addition to common oxides containing a single metallic element.
The above-mentioned shift of the chemical composition (hereinafter referred to as "composition") of the thin oxide film away from that of the oxide target is believed to be caused by the following reason:
(1) The negative oxygen ions sputtered from the oxide target are accelerated toward the underlying (e.g. substrate) surface because of a negative target self-bias voltage attributed to a difference in mobility between electrons and sputtering gas ions to allow the resulting accelerated oxygen ions to re-sputter those atoms with a large ion radius which are contained in the thin oxide film being formed; and
(2) The heating of the underlying surface for facilitating the crystallization of the thin oxide film being formed, causes atoms and/or molecules having a high vapor pressure to evaporate from the thin oxide film.
This compels the following procedure, for example, to be used in order to form a thin compound oxide film having a desired composition by sputtering. A thin film is first formed, using a compound oxide target having a given composition, to examine the composition of the thin film. Subsequently, a new thin film is formed, using another compound oxide target composed of the same components as those of the former target but having a different ratio in components to examine the, composition of the new thin film as well. This is repeated while by replacing the old target with a new target having a different ratio of components to approximate the composition of a thin film to the above-mentioned desired composition. Since this procedure requires the repetition of sputtering with each target replacement, large amounts of labor and time are needed to complete the formation of the thin film having a desired composition.
The conventional RF magnetron sputtering method using the industrial frequency of 13.56 MHz involves the following problems:
(1) A thin oxide film formed by sputtering using a target having a given composition differs greatly in composition from the target, as above described.
(2) The surface morphology of the resulting thin oxide film is not always good; and
(3) The compositional distribution across the whole surface of the thin oxide film is not always uniform.
The present invention has been made in view of the foregoing problems of the conventional method.
Accordingly, an object of the present invention is to provide a process for forming a thin oxide film. The film produced by the process of the present invention has a composition closer to that of the target, than the film from the conventional method, a more uniform across-the-surface compositional distribution as compared with the conventional method, and which may occasionally be excellent in surface morphology (flatness), unlike the film from the conventional method.